The present invention relates to an optical subscriber network transmission system, and more particularly to a network transmission system which is well suited to smoothly enlarge a transmission capacity when needed, for example, if information require substantial transmission capacity is offered to subscribers by the use of single-mode optical fibers.
An example of a network which offers various services with optical fibers, is shown in FIG. 1. Service information items are transmitted from a center 1 to individual subscribers 3 through optical fibers 2. As the contents of the services, there are mentioned conventional audio-telephone, and besides, facsimile, pay TV, high-definition TV, etc. A system of this type is described in, for example, "Integrated Circuit for Broad-Band Communications Systems" by H. Reiner, IEEE JOURNAL ON SELECTED AREA IN COMMUNICATIONS, VOL. SAC-4, NO. 4, JULY 1986, pp. 480-487.
The services can be classified into symmetric and asymmetric types. That is, the services such as telephone and facsimile in which the information rates of a transmission line from the center to each subscriber (downstream transmission line) and a transmission line from the subscriber to the center (upstream transmission line) are equal to each other are called the symmetric type, while the services such as pay TV and high-definition TV in which the capacity of the downstream transmission line is larger are called the asymmetric type.
In general, in an optical subscriber system, the symmetric and asymmetric services are multiplexed and then transmitted by the upstream and downstream transmission lines. Therefore, the transmission capacity of the downstream transmission line becomes larger than that of the upstream transmission line.
In addition, single-mode fibers of very broad band are employed for the transmission of signals between the center and the subscriber. Therefore, even if the subscriber should wish to subscribe in the future to services requiring still larger transmission capacity, only the transmitter and a receiver need be replaced with ones for higher capacity transmission of information, while the laid fibers are used as they are.
In particular, a transmitter and a receiver having been used for the downstream transmission line can be diverted to the upstream transmission line and a transmitter and a receiver of larger capacity can be installed anew for the downstream transmission line. This is more economic owing to the effective utilization of the existing facilities than a measure in which transmitters and receivers are installed anew for both the upstream and downstream transmission lines. This will be elucidated with reference to FIGS. 2(a) and 2(b).
FIG. 2(a) shows the connections between the center 1 and the subscriber 3 in the case where telephone and pay TV are subscribed to. Via the optical fiber 2-1 of the upstream transmission line, a voice signal from a telephone set 11 is transmitted to the center 1 by a transmitter 12 and is received by a receiver 13.
On the other hand, as regards the downstream transmission line, a voice signal is applied to a terminal 15, while a pay TV signal is applied to a terminal 16, and the signals are multiplexed by a multiplexer 17. The multiplexed signal is transmitted by a transmitter 18 and received by a receiver 19 via the optical fiber 2-2 of the downstream transmission line. The received signal is demultiplexed by a demultiplexer 20 into the voice signal and the pay TV signal, which are respectively applied to the telephone set 11 and a TV receiving set 21.
An example in which facsimile and high-definition TV are further added to this system, is shown in FIG. 2(b). In this case, the voice signal from the telephone set 11 and also a facsimile signal from a facsimile set 31 are multiplexed on the upstream transmission line by a multiplexer 32, whereupon the multiplexed signal is transmitted center 1. In the center, this signal is demultiplexed into the voice signal and the facsimile signal by a demultiplexer 33.
As regards the downstream transmission line, the multiplexed signal of the voice and pay TV signals applied to the terminal 51 of a multiplexer 54, a facsimile signal applied to the terminal 52 thereof and a high-definition TV signal applied to the terminal 53 thereof are multiplexed and then transmitted. The multiplexed signal is demultiplexed by a demultiplexer 57 on the reception side, and the demultiplexed signals are applied to the respective receiving sets.
In changing-over the system of FIG. 2(a) to that of FIG. 2(b), it is economical that the transmitter and receiver 18, 19 for the downstream transmission line of the system in FIG. 2(a) are diverted for the upstream transmission line of the system in FIG. 2(b). On account of the change-over work, however, the operation of the system needs to be stopped for a considerably long term. More specifically, in the system of FIG. 2(a), the transmitter 18 and the receiver 19 are detached and are respectively substituted for the transmitter 12 and the receiver 13, whereupon the transmission line is tested. Next, the multiplexers 32 and 33 are added to the transmission line as illustrated in FIG. 2(b), and the overall test of the upstream transmission line is conducted. In addition, a transmitter 55 and a receiver 56 are added anew to the optical fiber 2-2 of the downstream transmission line, and a transmission test is conducted. Thereafter, the multiplexer 54 and the demultiplexer 57 are added, and the overall test of the downstream transmission line is conducted. Lastly, the overall test of the upstream and downstream transmission lines is conducted. When the tests in such a procedure are collectively executed for several subscribers or ten odd subscribers, it is sometimes the case that interruption of service for a week or more is necessitated.